Envelope tracking (ET) is a known technique for improving the efficiency of power amplifiers. In a conventional implementation of an envelope tracking technique, a voltage signal at the drain input of a radio frequency (RF) power amplifier (PA) is varied to be proportional to the envelope of a RF signal. Tracking is performed in order to match the dynamic range of the supply voltage of the power amplifier to the instantaneous requirements of the RF signal envelope.
A subclass of the ET technique is the partial envelope tracking (PET). A conventional PET circuit replaces the tracking below a certain envelope voltage level, by providing a constant voltage of the power supply to the drain input of the RF PA. A conventional PET circuit also tracks the envelope peaks above that voltage level. One of the advantages of a PET technique is low power consumption and, specifically, low power consumption of a power supply when tracking signals with strongly varying envelopes. Examples for such signals include long-term evolution (LTE) signals, wireless local area network (WLAN) signals, and the like.
Different implementations for PET circuits are discussed in “Push the Envelope” to Kim, et al. (hereinafter “Kim”) published in IEEE Microwave Magazine, May 2013, which is incorporated herein by reference merely for the useful understanding of the background. The PET circuits discussed in Kim are complex and designed to operate mainly in base stations of cellular networks. Such PET circuits are designed to handle signals with strong envelope fluctuations in base station amplifiers with multicarrier systems. Further, in base stations, power amplifiers are assembled as stand-alone units and, therefore, components' size is typically not of major importance. Therefore, PET circuits, such as those discussed in Kim, are large in size and cannot be readily integrated in handheld devices.
Other implementations of PET circuits, discussed in the related art, are based on maintaining the Direct Current (DC) component of a power amplifier's current in the same path both during non-tracking as well as during envelope tracking intervals. Such circuits are based on chokes and transformers in the video coupling circuits, which maintain the DC component of the power amplifier's current.
Modern wireless communication systems are characterized by transmissions of RF signals with rapidly varying envelopes. The RF signals are characterized by high peak to average power ratio (PAPR). The use of PET circuits to provide mitigation for the varying envelopes and to improve the efficiency of an RF power amplifier (PA) is therefore required in such systems, and in particular wireless handheld devices. The power consumption of a battery used to supply the required power to the RF PA in such devices and the amount of heat dissipated due to the relatively low efficiency of the RF PA operating in the high PAPR regime, are major concerns, and as such drive the need for PET circuits.
The PET circuits discussed in the related art cannot be efficiently utilized in wireless handheld devices. Specifically, as noted above, the conventional PET circuits are large in size and primarily designed to support power amplifiers that are stand-alone modules. Further, standard implementations of PET circuits include inductors (or chokes). All of these properties do not meet the requirements of wireless handheld devices. Specifically, PET circuits in such wireless handheld devices should typically be implemented as integrated circuits (ICs), which must be small in size and must contain a minimal amount of external components and avoid the use of large components such as inductors. In addition, in handheld devices, power amplifiers are not necessarily stand-alone components, as a baseband signal generated by a baseband module is in the immediate vicinity of the amplifier.
Another major drawback of conventional PET circuits is that these circuits cannot efficiently handle high bandwidth signals. High bandwidth signals tend to fluctuate frequently with large slopes. Therefore, it is difficult to accurately track rapid changes in the envelopes of conventional PETs and to adjust the bias voltage of any PAs contained therein accordingly.
The conventional PETs operate via one path for handling both modes of operation: a normal mode and a tracking mode. The rapid changes in the signal's envelope require rapid switching between these modes. This rapid switching causes spurious effects in the spectrum and degrades the efficiency of the PET circuit.
It would therefore be advantageous to provide a PET solution that would overcome the deficiencies noted above and be efficiently implemented in wireless handheld devices.